Apparatus known as sensitometers have been used for many years to expose samples of photographic papers and films in a very precise manner for subsequent densitometric analysis in which the density of the image produced by such exposure is measured. Such apparatus and the associated methods must be capable of exposing the photosensitive sample with a high degree of precision, accuracy and repeatability. More particularly, excellent control must be achieved for factors such as the illuminance at the exposure plane or the amount of radiant energy per unit area on the sample at a given point in time, the exposure time or the period during which the sample is exposed to radiant energy, the color of such illuminance, the color temperature or spectral distribution of the radiant energy reaching the sample and the uniformity of the exposure across the surface of the sample. In the testing of photographic films and papers, a further concern is to be able to test the sample at illuminances, exposure times and color temperatures which closely approximate those to which end users subject the actual products. For testing purposes incident to manufacture of such films and papers, these three aspects of the exposure must be controlled more precisely than ever would be required by the end user. To maintain high productivity, some end users want to expose the product at faster and faster shutter times. To test a sample at such exposure times, the illuminance at the exposure plane must be increased in inverse proportion to the exposure time if the sample is to receive the same total amount of radiant energy as at a slower shutter speed. The flexibility to meet such changing test requirements has been largely absent from known sensitometric systems in which radiant energy requirements are high.
In known sensitometers, the radiant energy source typically is located far enough away from the sample that the sample is essentially uniformly illuminated and still receives enough radiant energy for proper exposure. Exposure times are created by a variety of shutter mechanisms, located either very close to the light source or very close to the sample. The radiant energy reaching the sample is attenuated through a test object or wedge or step tablet located between the source and the sample, with the sample usually pressed flat against the test object. Such test objects are made from a material transparent to the radiant energy to which have been added graded amounts of some spectrally neutral attenuating material, such as carbon or Inconel in the case of visible light, often in twenty-one individual steps measuring about 10 mm by 10 mm (0.39 by 0.39 in) and sandwiched between two pieces of color-clear glass. Thus, radiant energy transmitted through the test object is attenuated by the added material before striking the sample. Often, the exposed and processed sample has an exposed area measuring about 210 mm by 10 mm (8.27 by 0.39 in) which is made up of twenty-one contiguous steps, each step in the test object being made to attenuate radiant energy in a different proportion than either adjacent step. Such test objects typically attenuate visible light by 0.10 log, 0.15 log or 0.20 log increments to form what are called 0-2, 0-3 and 0-4 gradient test objects, respectively.
While such sensitometer apparatus and methods have long been used with acceptably good results, a variety of problems have been identified. Sensitometers that position the light source close to the exposure plane can significantly increase the energy incident on the exposure plane, but doing so can cause the distribution of energy at the exposure plane to be highly non-uniform. Those sensitometers that position the light source far from the exposure area in an attempt to uniformly illuminate the exposure plane typically deliver low exposure energy. Also, while distancing the light source from the exposure plane improves illumination uniformity, the uniformity can still be adversely affected by the non-uniform spatial emission characteristics of the light source itself. Since the spatial emission for each individual light source is often different, it can be very difficult to maintain exposure plane uniformity over a long period of time. Because of the way test objects are made, it is difficult to set with precision the degree of attenuation in each step and it is difficult to change the attenuation in any step once the object has been made. For the same reason, it is difficult to make any two objects just alike. Also, because these test objects are used during an exposure when physically touching the sensitized material, any dust resting on the test object, optical phenomena (e.g. Newton rings) or physical imperfections in the materials making up the test object are imaged directly onto the sensitized material, causing an undesirable non-uniform image. Also, nonuniformities in any optical element placed in the optical path during an exposure can produce a non-uniform as well as spectrally degraded image. Also, the use of a test object in contact with the sample during an exposure can make it difficult to measure the amount of energy illuminating the sample while the exposure is taking place.
One prior art sensitometer known to the present inventors includes a light source positioned only inches from an exposure plane containing a twenty-one step contiguous step test object and is designed to illuminate only a 10 mm step exposure format.
Another prior art sensitometer places a twenty-one step contiguous step test object at one end of an integrating chamber which is sized large enough to uniformly illuminate all of this test object with one exposure.